Electric vehicles (EV) cannot utilize engine exhaust heat for heating because they cannot be fitted with an internal combustion engine.
Also, hybrid electric vehicles (HEV), plug-in hybrid electric vehicles (PHEV), and similar hybrid electric vehicles with an engine are controlled to shut off the engine as much as possible for fuel-efficiency. Taking on board these issues, research is being done on heating by heat pump heating systems that use a refrigerant and electric heaters that use coolant as a medium.
The power consumed when heating is extremely large, and so heat pump systems, which have a high coefficient of performance (COP) for heating, have become desirable as a system for heating. The COP of electric heaters, in contrast, does not even reach a value of 1. However, heat pump heating at low outside temperatures (for example, −10° C. or below) may cause moisture in the air to condense and form as frost on the outside heat exchanger, potentially causing a decrease in the heating performance.
When frost forms on a room air conditioning device for household use or the like, the system operates in reverse (switches from heating operation to cooling operation) to remove frost from the outside heat exchanger. However, a similar frost removal operation in a heat pump system for a vehicle would result in the heating operation ceasing.
In order to solve the problem described above, Patent Document 1 has described a heating and cooling device for an automobile comprising a hot water refrigerant heat exchanger disposed on a refrigerant outlet side of an outside heat exchanger (main condenser) that functions as an evaporator. When the outside heat exchanger becomes frosted due to low outside air temperatures, the heating and cooling device for an automobile secures the necessary heating performance by heating the medium, supplementing the insufficient amount of heat taken in at the outside heat exchanger, at the hot water refrigerant heat exchanger and evaporating the medium.